The invention relates to a method for reducing the thermal load of an automatic transmission for a motor vehicle in an emergency operating mode.
The utilization of the intelligence in electronics finds its expression in motor vehicles having modern automatic transmissions in the use of an electronic transmission control (EGS) in which, to a great extent, the criteria known to be relevant are met for the customers such as driving comfort and driveability and also relevant to security. For the selection of a gear, according to situation, the electronic transmission control steadily communicates with other control units and computers of different aggregates via a CAN (Controller Area Network) data bus.
For the case of a defect in the electronic transmission control, emergency operating programs have often been provided which are designed so that in an emergency operating mode a vehicle can be operated in the widest possible driving range. But depending on the emergency running, the cooling of the transmission is problematic in such an emergency operating mode, since the heat accumulation in the emergency operating mode of the transmission must be eliminated, via the cooling system of the vehicle and, in case of complete failure of the electronic transmission control, the transmission has no possibilities of its own to positively act upon the thermal economy.
This particularly occurs in CVT (Continuously Variable Transmission) automatic transmissions which have a variator for continuous adjustment of a reduction ratio between a shortest possible ratio (LOW) and a longest possible ratio (overdrive, OD).
Such a CVT transmission with a first cone pulley pair upon an input shaft as a primary pulley set and with a secondary cone pulley pair upon an output shaft as a secondary pulley set has been described in xe2x80x9cATZ Automobiltechnische Zeitschriftxe2x80x9d 96 (1994). Each cone pulley pair consists of a first pulley axially stationary and a second pulley movable in axial direction which are designated as a primary pulley and a secondary pulley according to which of the primary or secondary pulley sets they belong. A torque-transmitting element which constitutes, e.g. a linked band, is wound around the cone pulley pairs. To adjust the primary pulley or the secondary pulley, they are loaded with a pressure medium from a pressure source, the pressure acting, respectively, on adjusting chambers of a primary pulley and a secondary pulley controlled with the aid of an electronic transmission control and an electrohydraulic pressure-supply device.
DE 44 36 506 has disclosed a device for control of a CVT transmission in which an electronic control unit determines, via electromagnetic actuators and hydraulic valves, the pressure level of the adjusting chambers of a primary pulley and a secondary pulley which are not dynamically balanced.
For a failure of the electronic control unit, an emergency operating device is provided which has two pressure-regulating valves, two pressure-reducing valves and at least one emergency valve, wherein a primary valve and a secondary valve adjust a constant static pressure ratio or power ratio between primary pulley and secondary pulley in constant static secondary pressure level.
The magnitude of the ratio change of the CVT transmission from a normal operation to the emergency operating mode can be established, via the amount of the static power ratio and the dynamic forces on the pulleys not dynamically pressure balanced. At the same time, the emergency operating mode is described as a state of the CVT transmission in which a rotational speed regulation and a contact regulation of the secondary pulley is interrupted.
In this known device, the ratio of the CVT transmission varies according to the generated torque of an internal combustion engine provided as input unit wherein the variable ratio change in the emergency operating mode makes a reliable start on the hill possible and an improved driveability at high top speed.
As input signals are supplied to an electronic control unit, for example the signal of a load position of the input unit like the throttle valve position in case of an internal combustion engine, the rotational speed of the transmission input shaft, the rotational speed of the output shaft, or the temperature of the pressure medium. From the input signal, an operating point is determined and the appertaining rotational speed of the transmission input shaft or the ratio of the CVT transmission adjusted. When an error occurs, the CVT transmission changes over to the emergency operating mode in which no rotational speed regulation and no contact pressure regulation of the secondary pulley no longer takes place.
Even through this known solution offers the widest possible adjustment range of the variator, it is problematic that the contact pressure for the corresponding pulleys must be designed according to extreme angular points of the driveability range. One of the angular points is the guaranty of the maximum starting power in the lowest gear, i.e. in or near the LOW ratio. For the variator capacity of transmission, a higher contact pressure on the secondary pulley is required at high ratio and high input torque. As result of the constant static pressure ratio between primary pulley and secondary pulley, the static force adjusted on the secondary pulley, whose magnitude depends on the maximum input torque to be transmitted, is constant in the whole torque range.
However, the consequence of this in wide driving ranges is an undesired overpressure, which disadvantageously leads to a high operating temperature in the variator with increased loss of effectiveness and wear of the variator system.
The use of a commuter electronic system is especially critical in this connection.
The problem on which this invention is based is to provide a method for reducing the thermal load of an automatic transmission of a motor vehicle in an emergency operating mode with which method the thermal and mechanical damages on the automatic transmission are prevented due to heat action during the emergency operating mode.
With the aid of the inventive method in which, in the emergency operating mode, an admissible maximum engine torque for the emergency operating mode (M_mot_max_not) is preset by the digital engine electronics system, it is possible to prevent, by a simple limitation of the operating range, any thermal overstress and heat-determined wear of the components of the automatic transmission, and thermal aging of the transmission oil.
The limitation of the engine torque by a program module in the engine electronics, which also leads to a limitation of the vehicle speed, and represents a simple but effective step, both in regard to an electrohydraulically controlled stepped automatic transmission of the conventional type and to a CVT transmission.
The use of the inventive method is of special advantage in CVT transmissions, since with only a slight limitation of the operating conditions, the generation of heat in the variator is prevented together with the attendant consequences of wear.
The inventive protection function also serves very advantageously protection in a converter automatic transmission in case the possible transmission input torque, as consequence of the torque overshoot, can exceed the admissible transmission input torque specific to the transmission part.
In this case, the maximum admissible transmission input torque on the part of the digital engine electronics system can be permanently protected by a fuse or by a corresponding limitation of the maximum admissible engine torque in the emergency operating mode.
It can also be alternatively provided that the digital engine electronics system, by means of a so-called stall speed limitation, carries out a self-sufficient engine engagement by which a converter increase is simultaneously limited.